Plasma processes for micro-machining, surface modifications, cleaning, sputter coating, and many other operations are widely used in the fabrication of micro-electronics. The ionized gas in a plasma generates a high concentration of reactive species at 50-100 C, and provides a means of cleaning, etching, and depositing materials at much lower temperature than is possible by thermally driven reactions alone. A plasma may be considered as an ionized gas consisting of a “chemical soup” of many types of species, such as positive and negative ions, electrons, neutrals, atoms, molecules, clusters, etc. In laboratories, plasmas can be easily generated by striking a high voltage (normally larger than 1 kV) electrical discharge through a low pressure gas system. This may be achieved using a DC, AC, or more usually, high frequency AC operating in the kHz-MHz (radio frequency, RF), or GHz (microwave) regimes.
As known to those skilled in the art, a plasma reactor includes a plasma chamber with inlet and outlet for gas entrance and exit, as well as a pair of electrodes distanced one from another. A voltage difference is created between the electrodes. The discharge is initiated by an electron that originates from either photoionization of a neutral species or field emission caused by strong electric fields around a sharp point at the electrode surface. The applied electric field between the electrodes in the plasma reactor accelerates this electron causing it to undergo scattering collisions with gas molecules. As the electron accelerates, eventually it will possess enough KE to ionize a gas molecule through an inelastic collision. This process releases another electron, which itself begins to accelerate, thus initiating a cascade mechanism whereby other gas molecules are ionized in subsequent collisions. To sustain the plasma, electrons must be generated at a rate which is large enough to offset the loss of electrons to the chamber walls, recombination with positive ions and/or electron attachment reactions. Another mechanism which generates electrons is bombardment of the electrode surfaces by energetic particles (ions or electrons), which causes high energy secondary electrons to be ejected back into the plasma.
One of the features of a plasma is that while the bulk of the glow discharge remains at equipotential, most of the voltage applied between the electrodes is dropped across a small region surrounding any surface in contact with the plasma, and in particular the electrodes. This region is called the sheath (or “dark space”). The positive ions from the bulk of the plasma can be accelerated across this sheath and strike the electrode with high KEs. The voltage through which the ions are accelerated, and hence the KE with which they strike the substrate, depends upon many factors, including the applied power, the gas pressure in the plasma reactor, and the chamber geometry. As a consequence, a variety of plasma systems have been developed to exploit different ranges of these properties for different applications, specifically in microelectronics for fulfilling manufacturing steps, such as surface modification, etching, deposition, doping, etc.
Nanoscale processing of thin films and substrates by exposure to electrically charged plasmas is a desirable technology for surface modification and other changes of substrates. For plasma processing of nanoscale sized features, such as thickness, depth, and lateral dimensions, the control of the interaction time between the plasma and the work piece is of great importance. Typical etching or deposition rates using plasmas range from a fraction of nm/s to many nm/s. Because of the nanoscale thickness of the material layers that need to be added, modified or removed, the plasma/substrate interaction time has to be short. Simultaneously, a fully established plasma needs to interact with the work piece, rather than a plasma that is still evolving as a function of time, e.g., during the initial transient. This is especially important for chemical reactive discharges.
Current technology suffers from several disadvantages when applied to plasma processing of nanoscale layers. For instance, it is common that the substrate is in contact with the plasma during all phases of the plasma process such as initialization of the plasma, biasing of the substrate, desired plasma processing by plasma/substrate interaction, and plasma extinction. The beginning and the end of the desired plasma/substrate interaction are not well-defined in this approach. This is acceptable for the processing of thick layers, but not acceptable for plasma processing of layers with nanoscale dimensions. Any changes of the substrate introduced by the inadvertent interaction of the substrate with the plasma during one of the undesirable periods, such as initialization of the plasma, biasing of the substrate, stabilizing plasma, plasma extinction, and decay of long leaved neutrals, may reduce the efficacy of the plasma process. As an example, in a fluorocarbon (FC) based plasma etching process used to transfer lithography defined features into a film, fluorocarbon film deposition takes place after the plasma has been ignited but the RF bias has not been applied to the substrate electrodes. This deposition process can have unacceptable consequences for profile control of ultra-fine features.
Another drawback of the current plasma processing, is that the etching depth and/or film thickness of deposited films is not well controlled because of the sequential and additive effects of different regimes of the plasma/surface interaction. For plasma etching, this is dealt with by employing etch stop layers, in order that the plasma processing time can be increased to a period sufficient to guarantee the complete removal of the subject film. Obviously, this approach increases process complexity, and for plasma processing of substrates with nanoscale dimensions may be impractical, if not impossible.
In order to attain addition or removal of precisely controlled layer thicknesses of materials (nanoscale dimensions), tightly controlled plasma/surface interaction is desired. The attempt has been made in the plasma processing art to enhance process control. For example, U.S. Pat. No. 6,132,805 teaches the use of a moving shutter positioned within the processing chamber at a distance from a substrate. The shutter is activated to interrupt plasma/substrate interaction when needed. A plasma can be generated between the shutter and the substrate for the substrate cleaning. A second plasma exists between the shutter and a remote electrode. With the shutter initially closed, both plasmas are established. After a certain amount of time, the shutter is removed, and the second plasma is allowed to interact with the substrate for the deposition of a thin film.
The disadvantage of the thin film processing equipment described in the U.S. Pat. No. 6,132,805, is that the shutter is located an extended distance from the substrate, and the system uses the plasma generation between the shutter and the substrate. This is undesirable for nanoscale plasma processing where the exposure time of the substrate to any kind of plasma or reactive environment must be precisely controlled. This control of the short time interaction of a substrate with a fully established and stable plasma is not possible in the system described in the U.S. Pat. No. 6,132,805 since the existence of a large volume between the shutter and the substrate causes appreciable changes in the plasma once the shutter is removed from the substrate for processing.
Another deficiency of the prior plasma processing techniques is that if a microscopic shutter is used to control the exposure time of the substrate to a plasma, either by movement of the shutter or of the substrate relative to the shutter, a sheath will form at the surfaces of the shutter. This sheath changes both the energies and angles at which incident ions bombard the surface of the substrate in the shutter/sheath area. The problem is that orientation of the sheath may be directed parallel to a normal of the substrate which prevents the ion bombardment of the substrate. The use of the microscopic shutters in the plasma processing becomes problematic if a very short time exposure of a substrate to the plasma is required, i.e., for nanoscale layer processing or for diagnostics of the plasma. For such conditions the interaction of the substrate with a perturbed species and energy flux from the plasma may become dominant. It is therefore desirable to develop a plasma processing technique in which the effect of the sheaths surrounding the shutter in contact with plasma will be obviated.
The plasma processing for nanoscale micro-electronics with an enhanced process control which would allow a precisely controlled short time interaction of a substrate with a fully established and stable plasma is therefore a long lasting need in nanoscale device fabrication.